20IMPACTS Tropical cyclone Kay remnant moisture and Tropical Storm Hilary landfall with flash flooding, and debris flows in Southern California

Tropical cyclone Kay moved northward along the Mexico and California Baja September 7-9, 2022 before veering to the west between 50 to 100 miles south west of San Diego. Precipitation bands ahead of the cyclone moved across Southern California on September 9 producing widespread rain and strong winds due to the south to north oriented mountains in San Diego County. The enhanced offshore flow from the tropical cyclone resulted in downsloping wind gusts, which were measured between 60 and 110 mph. There was also heavy rainfall enhanced on the normally dry side of the mountain ranges or the east slopes of deserts. Between 2 and 6 inches of rain occurred across the Imperial Valley into the San Diego mountains east slopes The rain bands moved out on September 10 leaving tropical cloud cover and deep moisture which eventually yielded to partial sunshine on September 11 and 12th. The tropical moisture and sunshine resulted in deep instability across Southern California and developed numerous thunderstorms from the mountains to near the coast moving slowly east to west in the tropical cyclone remnant steering flow. On September 11 and 12th, several thunderstorms produced copious amounts of rainfall and massive debris flows and flash flooding in the San Bernardino mountains, including the Apple and El Dorado burn scars. Four separate flash floods and debris flows occurred including the entire San Gorgonio river, which brought millions of dollars of damage to a water facility in Banning. Other debris flows occurred in Forest Falls, resulting in a fatality of a 60 year old female and extensive search and rescue efforts as well as damage to several homes. A homeowner captured the rock debris flow and flash flood as it surrounded their home, and swept away one of their vehicles. A separate debris flow occurred in Oak Glen, pushing 4 feet of layered mud into a restaurant and homes in the Birch Creek drainage. The debris flow was caught on a San Bernardino county public works security camera. Thousands of cubic yards of debris were removed from the affected areas. Rainfall rates of 1 to 2 inch per hour were recorded along the higher elevations of Yucaipa Ridge in the lower San Gorgonio mountain peak.

Prior to the tropical system the hottest day on record for California occurred September 5, 2022 in the middle of a record two week heat wave. At the start of the heat wave NWS office in San Diego provided enhanced decision support services (DSS) for the excessive heat, increased fire weather risks and heavy rainfall potential. This DSS led to onsite support at the Riverside County Emergency Management Department as heat health impacts, wildfires and then flash flooding unfolded across southern California. The timeline of the weather events and impact DSS will be outlined in this presentation.

At the time of this abstract Tropical Cyclone Hilary is projected to make landfall in San Diego and could bring major impacts, therefore the author will also present some information on this unusual event.